It is known to provide camshafts with targets for rotational speed and position sensors and a prior art example can be found in GB-2317958. In this arrangement, the camshaft sensor target is formed in one piece with the camshaft itself. This requires machining operations to be carried out on the camshaft so as to produce the target lobes from solid.
Instead of forming the sensor target in one piece with the camshaft, in some arrangements a camshaft sensor target is formed as a separate component and is then attached to the camshaft for co-rotation. Examples of such arrangements may be found in U.S. Pat. Nos. 5,627,464, 5,987,973 and in U.S. Pat. No. 6,277,045. In each of these cases, a separate component incorporating a camshaft sensor target is attached to an end of the camshaft using a threaded fastener.
In many instances of camshaft rotational speed and position sensing, it may be noted that accuracy of the whole arrangement is very sensitive to variations in the air-gap between a sensor and its target on the camshaft. The width of the sensor-to-target air-gap is often dependent on a tolerance stack that comprises essentially two components. The first part is the tolerance stack built up in making the sensor itself and putting it into position, often on a bracket or boss on the cylinder head or cam-cover. The second component is the tolerance stack in making the target, fitting it to the camshaft and in putting the camshaft into place with due consideration to running clearances and wear in service. When the need arises to provide a camshaft sensor target on a portion of a camshaft that is not solid, particular problems may arise in relation to distortion of the target and/or the camshaft, with subsequent adverse effects on the associated part of the tolerance stack.
It is also known to construct composite camshafts and a recent example of such a prior art camshaft is disclosed in U.S. Pat. No. 6,182,361. In this particular arrangement, camshaft lobes and journal elements are made as components that are initially separate from a tube forming the basis of the camshaft. The lobes and journals are then pushed onto the tube and fixed in place by a permanent interlock. The preferred approach is to stake or crimp the lobes and journals in place, with alternatives of welding and brazing being suggested. While the suggested methods of interlocking may be acceptable for cam lobes and bearing journals, it should be born in mind that these are fairly sizable parts.
If a target arrangement for camshaft rotational speed and position sensing is to be provided on a portion of a camshaft that is not solid, it may prove difficult to hold it in position with a sufficient level of accuracy. This is particularly so if the target used is formed from a thin plate. For example, with a hollow portion of a camshaft, staking in place a pre-formed target by crimping it to the hollow portion may result in crush deformation of the target, the shaft or both. That in turn may cause variations in sensor-to-target air-gap tolerance that are unacceptable. Similar problems may arise from heat joints such as welding or brazing due to distortion on heating or cooling and shrinkage. In addition, such heat joints call for complicated production methods and equipment. The thinner the material from which the target is made, the greater will be the risk of distortion. Furthermore, in fixing a separate target member to a hollow portion of a camshaft, it is apparent that use of a mechanical fixing such as a threaded fastener may not be practical.
Similar problems, at least in alignment, may occur in cases where the camshaft element is one configured for transferring drive to or from the camshaft. Such an element may comprise a gear wheel or pulley for “V” belts or toothed drive belts.
There is therefore a continuing need to apply camshaft elements such as sensor targets to camshafts and to do so with good build consistency in high volume applications. It is also apparent that this need may be particularly difficult to satisfy if such a camshaft element is to be applied to a portion of a camshaft that is hollow.